The present invention relates to a process for the production of glass-like carbon (GC) substrates for use as recording media, as well as a setter for use in the process.
Aluminum has commonly been used as a material for the substrates of recording media typified by hard disks. In recent years, with a view to meeting various requirements such as higher impact resistance, lighter weight realized by reduction in thickness, and smaller power consumption by motors realized by reduction in weight, manufacturers are getting interested in "GC substrates" which are made of GC.
A conventional process for the manufacture of GC substrates is illustrated in FIGS. 9A to 9D. The process starts with a resin synthesis step, in which a phenol-modified furan resin is synthesized in a reaction vessel 1 and the synthesized liquid resin is filtered through a filter 2 (FIG. 9A). The process goes to a resin curing step, in which the liquid resin loaded with a curing agent is injected between a pair of glass plates 3 and left to stand at high temperature to effect a curing reaction (FIG. 9B). The cured resin is removed from a clearance between the glass plates 3 as a sheet 4 of a certain size.
The next step is core making, in which e number of disks 5 each having a center hole are cut out of the sheet 4 by a suitable means such as a laser cutter (FIG. 9C).
In the subsequent carbonizing step, a plurality of disks 5 (cured resin substrates) are placed side by side on a "setter" 6 (a flat plate typically made of graphite) in such a way that no adjacent disks will contact each other; another setter 6 is placed over the arranged disks 5 and another group of disks 5 are placed side by side on that setter in such a way that no adjacent disks will contact each other (FIG. 9D); a stack of setters 6 that support a specified number of disks 5 are placed in a baking furnace 7 (see FIG. 10) and baked at 1,000.degree.-1,500.degree. C. by a suitable means such as a graphite heater 8 in an inert gas (e.g. N.sub.2 or Ar, etc.) atmosphere, whereby the cured resin is carbonized to yield GC substrates.
The cured resin substrates (disks) 5 placed between setters 6 should not contact one another; if adjacent disks 5 remain in contact with each other during baking, they will fuse together.
The GC substrates are given the flatness and surface roughness necessary for the final products through the sequence of a surface lapping step, an exterior/interior chamfering step, and a final polishing step.
The conventional process for the manufacture of GC substrates has had several problems. The performance of baking furnaces (e.g. an electric furnace and a gas furnace) is generally determined by the effective inner volume of the furnace and the total weight of the workpieces (cured resin substrates or disks) and the setters (flat plates). In the conventional process, a plurality of workpieces are baked as they are placed side by side between setters in such a way that adjacent workpieces do not contact each other. If the setters are made of graphite, their specific gravity (.rho.=1.3-1.9) is greater than that of the workpieces which are made of a resin (.rho.=1.1-1.4); additionally, the total volume of the setters is greater than that of the workplaces. Thus, it is no exaggeration to say that the setters rather than the workpieces are baked and, as a matter of fact, the baking furnace is not operated to its full capacity.
The conventional practice of arranging a plurality of disks between setters in such a way that adjacent disks will not contact each other has another problem; that is, if there are variations in thickness between disks, the thinner disks will move sidewise to contact adjacent disks to cause fusion as a result of baking.
In an alternative approach, the disks are placed alternately with spacers but this is not problem-free, either. The disks are not completely fixed in the position where they are stacked and, hence, the applied load is not distributed uniformly among all disks and warpage and other defects often occur in the baked disks.